Toxoplasma gondii is capable of infecting virtually all types of warm-blooded vertebrates and frequently causes food or waterborne infections in humans. Although most human infections are normally uncomplicated, they persist in a chronic state and can predispose individuals to subsequent reactivation. Current therapies can suppress parasite growth but not eliminate persistent infection, and available treatments are limited by adverse effects. As such, there is a need for increased understanding of basic parasite biology to identify new targets for therapeutic intervention. Like other apicomplexan parasites, T. gondii is an obligate intracellular parasite, and cell entry is critical for its survval. Host cell invasion is governed by calcium-regulated secretion of adhesive proteins that are discharged from micronemes, together with the concerted action of an action-myosin motor that lies beneath the plasma membrane. Although previous studies demonstrate that calcium- regulated microneme secretion is essential, the molecular details of this signaling cascade are incompletely understood. Additionally, calcium is likely to control other important aspects of parasite biology. The goal of our studies is to identify key regulatory steps in calcium signaling that are essential for survival of apicomplexan parasites. We will utilize T. gondii as a model due to its versatility for genetic, cellular, and molecular studies. Prior studies implicate protein kinase G (PKG) in controlling calcium increases and activating calcium-dependent protein kinases (CDPKs) in apicomplexan parasites. The proposed studies will test the role of guanylate cyclases in producing cyclic GMP (cGMP) to activate PKG as part of this pathway. One of the unique features of apicomplexans is that they contain a family of CDPKs that are not found in their mammalian hosts. In previous studies, we demonstrated that microneme secretion requires the activity of several canonical CDPKs, which play partially overlapping roles in controlling invasion and egress. We will extend our studies to encompass the non-canonical CDPKs encoded in the T. gondii genome. These non-canonical CDPKs differ in the number and arrangement of calcium-responsive motifs and they contain additional domains likely to govern location and/or regulation. To support these studies, we have developed novel reporter strains of T. gondii that have been engineered to detect elevated cGMP, increased calcium, and microneme secretion in response to agonists. We will use molecular, cellular, and biochemical methods to examine the roles of guanylate cyclases and non-canonical CDPKs in controlling calcium-mediated signaling in T. gondii. Efficient methods for gene disruption or inducible deletion will be used to determine the roles of individual genes in intracellular survival. Collectively, the proposed studies will define the molecular basis of calcium signaling and elucidate the roles of members of a novel family of calcium-dependent protein kinases.